Cyclical propellers have been used for propulsion, where they are most commonly known as Voith Schneider propellers, and as windmills, where they are commonly known as vertical axis wind turbines (VAWT) or Darrieus rotors. Another commonly used term is cycloidal propeller, which is named for the path that a blade of the device follows for one particular range of ratios between flow speed and propeller rotation speed. This document adopts the more general term of cyclical propeller, which refers to propellers and turbines with one or more blades that exhibit periodic or variable pitch changes during rotations.
U.S. Pat. No. 1,835,018 to Darrieus in one of the first references to cyclical propellers, describes a rotating disc with one or more blades attached at a radial offset from the shaft of the propeller. FIG. 1 shows one such propeller 100 that mechanically varies the pitch angles of blades 110 as described in U.S. Pat. No. 1,835,018. Blades 110 of propeller 100 have pivot axes 120 running perpendicular to a base 115 and parallel to a central shaft 130 of propeller 100. A mechanism 140 driven by central shaft 130 and attached to blades 110 cyclically changes the pitch of the blades, i.e., the angle between each blade 110 and a line extending from shaft 130 to the pivot 120 of the blade 110. In propeller 100, the variation of the attack angle of blades 110 gives propeller 100 a direction of thrust when blades 110 are all submerged in the fluid. This type of propeller is able to produce torque at shaft 130 for any fluid flow direction passing parallel to base 115 and has been used successfully in vertical axis wind turbine installations.
The ability to extract energy from a fluid flow passing in any direction perpendicular to a main shaft of a propeller is a very important and desirable property for power extraction when the fluid flow direction can vary. The Darrieus' cyclical propellers have this ability and are also relatively simple and therefore inexpensive to construct and operate. However, the Darrieus cyclical propeller lacks a control system and will typically not start to rotate on its own from rest, but rather requires external power to start rotation. This makes such propellers unsuitable for wave power extraction, where the flow velocity varies between zero and maximum velocity for each wave passage. Further, since the maximum blade pitch of these propellers is fixed by the eccentricity between mechanism 140 and the main shaft 130, the Darrieus propeller cannot adjust or adapt to flow fields of different velocities. The propeller will thus have optimal energy conversion ratio for only one flow speed and greatly reduced efficiency if operated at a flow speed different from the optimal flow.
Currently systems for extracting power from waves have mainly relied on an oscillating water column, buoys, or similar floats that move up and down with the passing of each wave. These wave power converters generally have energy conversion efficiencies that are theoretically limited to being less than 50%. More efficient wave energy converters that can approach 100% efficiency are thus sought.